Abstract Ulcerative Colitis (UC) and Crohn?s Disease (CD) are the two most common Inflammatory Bowel Diseases (IBD) involving excessive and chronic inflammation of the gastro-intestinal tract. Currently, standard of care includes aminosalicylates, corticosteroids, general immunosuppressants and antibiotics with the aim to induce and maintain remissions. If remission cannot be achieved or the disease is too severe, biologics are administered. Nevertheless, roughly 60% of patients go into remission after treatment with biologics, and roughly 20% of these patients stop responding to therapies, indicating that numerous patients need alternatives to current therapies. Nicotinamide adenine dinucleotide (NAD) is one of the key coenzymes regulating many metabolic pathways. CD38 is a NAD-degrading ectoenzyme that plays a major role in NAD metabolism. In animal models CD38 modulates immune responses of T cells, macrophages and neutrophils. These models support the hypothesis that colonic inflammation leads to a decrease in NAD levels of intestinal cells. Subsequent NAD decline would decrease the activity of anti-inflammatory NAD-dependent deacetylases. Current mAbs against CD38 kill CD38+ cells and have been shown to be effective treatments of Multiple Myeloma. However, for auto-immune patients, removal of CD38+ cells, including suppressive immune cells, could lead to exacerbation of disease. We will use our fully human heavy chain only antibodies (UniAbs) to create multivalent/biparatopic molecules that solely block CD38 hydrolase activities. Our custom TeneoSeek discovery pipeline enables effective UniAbs screens by applying high-throughput functional assays. UniAbs will be expressed on silenced IgG backgrounds to eliminate immune effector functions. Preliminary results, using an in-house UniAb specific for mouse CD38 hydrolase, indicate that inhibition of CD38 hydrolase leads to higher intracellular NAD levels in in vivo mouse models and better clinical scores in DSS model, widely used IBD model in mice. Specific Aim 1: Identification and characterization of high-affinity blockers of human CD38 hydrolase activities. We will immunize our UniRats with recombinant human CD38 proteins with the goal to identify at least 2 UniAb sequence families with potent hydrolase blocking activities. Two sequence families will be selected that do not compete for binding to CD38. Specific Aim 2: Development of human bivalent and tetravalent high-affinity UniAbs. Combinations of VH domains selected in SA1 will be expressed as bivalent and tetravalent UniAbs. We will produce 0.5g of each hydrolase blocker for future animal studies. In phase II, pre-clinical studies will be performed on selected UniAbs to enable the filing of an IND. These studies will include tox and PK/PD studies in monkeys and mice. Proof-of-principle provided by this work will support development of the same product for other inflammatory diseases such as rheumatoid arthritis and asthma.